Nanotubes consisting of one wall are called single wall carbon nanotubes (SWCNTs) and have diameters ranging from about 0.4 nm to about 10 nm. SWCNTs are relatively rare and difficult to produce and characterize. Many theoretical studies have focused on SWCNTs because of their uniform and relatively simple atomic structures. In general, there is excellent agreement between theory and experiments on the various effects of atomic structure and the diameter of the nanotube on, for example, electron transport (conducting and semi-conducting nanotubes) and elastic properties.
Multi-walled carbon nanotubes (MWCNTs) include concentric nanotubes with an inter-wall spacing of about 0.34 nm and outside diameters on the order of about 10 to about 100 nm. MWCNTs have gained increasing interest in various industrial and technological applications because of the development of techniques to produce bulk quantities of high-quality nanotubes with uniform diameters, number of walls, and atomic structure.
There have also been extensive experimental studies on the ability of MWCNTs and other carbon nanomaterials to “store” hydrogen (H2) at low pressure (on the order of 0.01 GPa or less) for the purpose of containing the gas safely in automobile fuel tanks. However, low pressure “storage” is very different than the high-pressure storage as discussed herein. Low pressure “storage” mainly refers to an equilibrium distribution of relatively low-pressure hydrogen gas in and around the materials of interest and causes little or no stress in the nanotubes or nanotube bundles.
The first reported experiments that demonstrated the storage capability of MWCNTs were performed on the confinement of argon. In these experiments, bundles of MWCNTs were subjected to argon gas at a pressure of about 0.17 GPa at a temperature of about 650° C. for 48 hours. Upon releasing the sample to atmospheric pressure, some of the nanotubes were found to retain argon at pressures of about 0.06 GPa. In another work, the confinement of SF6, carbon dioxide, and 13CO2 in SWCNT bundles was studied. Open-ended SWNTs were charged with gas cryogenically, and the ends were subsequently closed by functionalization with ozone. The samples were stabilized in vacuum for at least 24 hours. After stabilization, they were found to desorb the retained gas upon heating to 700° C. in vacuum which indicated the various gasses were confined to the interior of the nanotubes.